Method and device for estimating parameters of a system for spreading the spectrum of a clock signal

Abstract

A method to estimate parameters of a system to spread a spectrum of a first periodic signal according to a modulation period. An embodiment comprises sampling the first signal using a second periodic signal, determining based on the sampling result each occurrence where the first and second signals are synchronous, incrementing a first counter at each sampling, the first counter being reset at each said occurrence, storing at each said occurrence the last value of the first counter before the resetting, providing a third periodic signal at a first level when said last value is greater than a threshold and at a second level when said last value is smaller than the threshold, and determining the modulation period based on the period of the third signal.

The invention claimed is: 
     
       1. A method, comprising:
 estimating parameters of a spreading system to spread a spectrum of a periodic first signal according to a modulation period, the estimating including: 
 sampling the periodic first signal using a periodic second signal; 
 determining, based on the sampling, each occurrence where the periodic first signal and the periodic second signal are synchronous; 
 incrementing a first counter at each sampling; 
 resetting the first counter at each said occurrence; 
 storing at each said occurrence a last value of the first counter before resetting the first counter; 
 providing a periodic third signal at a first level when said last value is greater than a threshold and at a second level when said last value is smaller than the threshold; and 
 determining the modulation period based on a period of the periodic third signal. 
 
     
     
       2. The method of  claim 1  wherein the spreading system is further defined by a modulation ratio, and the method further comprises:
 incrementing a second counter at each said occurrence; 
 incrementing a third counter at each said occurrence according to a comparison of said last value with the threshold; and 
 determining the modulation ratio based on a ratio of counts of the third counter to the second counters. 
 
     
     
       3. The method of  claim 2 , further comprising:
 determining a first time period corresponding to a determined number of cycles of the periodic second signal; 
 determining a second time period corresponding to a determined number of cycles of a periodic fourth signal at a same frequency as the periodic second signal and having no jitter; and 
 correcting the modulation period and the modulation ratio based on a ratio of the second time period to the first time period. 
 
     
     
       4. The method of  claim 1 , further comprising:
 providing a result of the sampling to an input of a shift register triggered by the periodic second signal; 
 determining a first quantity corresponding to a number of bits of a current word formed from parallel outputs of the shift register in a first state and a second quantity corresponding to a number of bits of the current word in a second state; and 
 counting a number of occurrences of an equality of the first and second quantities. 
 
     
     
       5. The method of  claim 1 , further comprising:
 providing a result of the sampling at an input of a shift register triggered by the periodic second signal; 
 comparing at least first states and last states of a current word formed from parallel outputs of the shift register with respect to at least one reference word; and 
 counting a number of occurrences of the reference word. 
 
     
     
       6. The method of  claim 5  wherein first and last bits of the reference word are in inverse states. 
     
     
       7. The method of  claim 5  wherein the reference word comprises first and second bit halves, each comprising a same number of bits in a first state and a same number of bits in a second state. 
     
     
       8. The method of  claim 5  wherein a number of bits of the reference word ranges between four and twelve. 
     
     
       9. A device to estimate parameters of a system to spread a spectrum of a periodic first signal according to a modulation period, comprising:
 a shift register having an input configured to receive the periodic first signal, sampled by a periodic second signal; 
 a first circuit configured to determine based on the sampling, each occurrence where the periodic first signal and the periodic second signal are synchronous; 
 a first counter configured to count each sampling and to reset at each said occurrence; 
 a memory configured to store a last value of the first counter before the resetting of the first counter at each said occurrence; 
 a second circuit configured to generate a third binary signal at a first level when said last value is greater than a threshold and at a second level when said last value is smaller than the threshold; and 
 a third circuit configured to determine the modulation period based on a period of the third signal. 
 
     
     
       10. The device of  claim 9  wherein the spreading system is further defined by a modulation ratio, and further comprising:
 a second counter configured to count each said occurrence; 
 a third counter configured to count each said occurrence when said last value is greater or smaller than the threshold; and 
 a fourth circuit configured to determine the modulation ratio based on a ratio of counts of the third counter to the counts of the second counter. 
 
     
     
       11. A system, comprising:
 means for generating a spread spectrum clock signal; and 
 means for estimating a modulation period of the spread spectrum clock signal, the means for estimating having:
 means for sampling the spread spectrum clock signal using a sampling clock signal; 
 means for tracking occurrences of synchronization between the spread spectrum clock signal and the sampling clock signal; and 
 means for generating a binary signal based on the tracking of occurrences of synchronization. 
 
 
     
     
       12. The system of  claim 11  wherein the means for sampling comprises a shift register. 
     
     
       13. The system of  claim 12  wherein the means for tracking comprises a pattern detector. 
     
     
       14. The system of  claim 13  wherein the means for tracking comprises a counter. 
     
     
       15. The system of  claim 11  wherein the means for generating a binary signal comprises a comparator. 
     
     
       16. The system of  claim 11 , further comprising means for estimating a modulation ratio of the spread spectrum clock signal. 
     
     
       17. A system, comprising:
 a spread spectrum clock generator configured to generate a spread spectrum clock signal; and 
 a modulation period estimator having:
 a sampler configured to sample the spread spectrum clock signal using a sampling signal; 
 a synchronization detector configured to detect occurrences of synchronization between the spread spectrum clock signal and the sampling signal; and 
 a modulation period detector configured to estimate a modulation period of the spread spectrum clock signal based on the detected occurrences. 
 
 
     
     
       18. The system of  claim 17  wherein the sampler comprises a plurality of flip-flops. 
     
     
       19. The system of  claim 17  wherein the synchronization detector comprises a shift register and a pattern detector. 
     
     
       20. The system of  claim 19  wherein the modulation period detector comprises:
 a sampling counter configured to count cycles of the sampling clock; 
 a memory configured to store an output of the sampling counter; and 
 a comparator configured to compare the stored output of the sampling counter to a threshold and generate a signal indicative of the modulation period based on the comparison. 
 
     
     
       21. The system of  claim 20  wherein the modulation period detector further comprises a counter configured to count transitions of the signal indicative of the modulation period from a low state to a high state. 
     
     
       22. The system of  claim 20 , further comprising:
 a modulation ratio estimator configured to estimate a modulation ratio of the spread spectrum clock signal. 
 
     
     
       23. The system of  claim 22  wherein the modulation ratio estimator comprises:
 a synchronization occurrence counter configured to count the detected occurrences of synchronization; 
 a threshold counter configured to count occurrences of the stored output of the sampling counter being less than the threshold; and 
 a divider coupled to the synchronization occurrence counter and to the threshold counter. 
 
     
     
       24. The system of  claim 17  wherein the modulation period estimator is configured to compensate for jitter in the sampling signal. 
     
     
       25. A method, comprising:
 estimating a modulation period of a spread spectrum clock signal in an electronic device, the estimating including:
 sampling the spread spectrum clock signal with a sampling clock signal; 
 tracking occurrences of synchronization between the spread spectrum clock signal and the sampling clock signal; 
 generating a periodic signal based on the tracking of occurrences of synchronization; and 
 estimating the modulation period based on a period of the generated periodic signal. 
 
 
     
     
       26. The method of  claim 25  wherein,
 the tracking the occurrences of synchronization comprises counting the occurrences of synchronization; and 
 the method comprises estimating a modulation ratio of the spread spectrum signal based on the counting of the occurrences of synchronization.